Numbuh 19th Century was a KND operative who lived during the 19th Century (as his name implies). He was the one who was sent into the Alamode to retrieve the secret cake recipe of the Annoyingly Cute Triplets Who Lived Upon the Hill. However, when he found it, he accidentally triggered a booby trap that set off a nuclear ice cream explosion that froze him and the five thousand other KND operatives present at the Alamode in ice cream. Within ten years, all but Numbuh 19th Century had been thawed out, but since they were now chronologically over thirteen, they were all decommissioned.
Extra Information: Assuming that exactly 200 years has passed since being frozen, and knowing that this particular episode aired in 2006, the year Numbuh 19th Century would have last existed in would have been 1806 - where he was originally ten years old. With this, subtract ten years and you'd have his approximate birthdate of 1796; he was born just before the turn of the 19th Century. Also, if he was still ten years old by the time he was unfrozen in 2006, he would be 15 years of age in 2011 - his accepted birthday is November 12th.
Click here for his other page on this website if you would like to know what happened after the episode.